1. Field of the invention
This invention relates, in general, to air conditioning apparatus including an internal heat-exchanger and a relatively large external heat-exchanger for cooling/heating a relatively large space. In particular, the invention relates to detection of the condensation temperature of such a large external heat-exchanger. The invention also relates to a method for controlling the rotation speed of an external fan device which provides air to the external heat-exchanger in accordance with the detection result.
2. Description of the related art
As shown in FIG. 1, a conventional heat-pump type air conditioner typically includes a compressor 11, a four-way valve 13, an internal heat-exchanger 15, a decompression device (e.g., expansion valve) 17, and an external heat-exchanger 19 connected in series by a fluid pipe 21. An external fan 23 and an internal fan 25 are respectively disposed opposite to corresponding heat-exchangers 15 and 19. A sensor 27 is attached to pipe 21 on one side of external heat-exchanger 15, which is the refrigerant discharge side during cooling. Sensor 27 detects the condensation temperature of external heat-exchanger 15. External fan 23 is controlled by a control circuit 29 in response to a detection signal. The detection signal fed from sensor 27 indicates the condensation temperature. Internal fan 25 also is controlled by control circuit 29 in response to temperature changes in a defined space to be air conditioned.
In such a conventional air conditioner as described above, a large external heat-exchanger is needed to enhance heat-exchange efficiency. However, flow resistance of the heat-exchanger increases because of the long serial fluid passage used in such large external heat-exchangers, and therefore, the flow rate of refrigerant decreases. To solve the above-described problem, the long serial fluid passage of the large heat-exchanger is divided into several passage elements 31a, 31b and 31c, as shown in FIG. 2. Each passage element 31a, 31b, 31c normally is arranged in a vertical direction. One end (intake side during cooling) of each passage element 31a, 31b, 31c is connected to a header pipe 33. The other ends (discharge side during cooling) of each passage element 31a, 31b, 31c are commonly connected to a collector 35. The above constructed external heat-exchanger 15 is connected to the refrigerating circuit shown in FIG. 1 through a pair of connection valves 37. Therefore, the passage elements 31a, 31b, and 31c are connected in parallel with one another. A temperature sensor 39, e.g., a thermistor, is disposed at the other end of the lower-most side passage element 31c, which is the discharge side during cooling, to detect the defrosting state during the defrosting operation.
In the above-described conventional air conditioner including a relatively large external heat-exchanger which has a plurality of path elements the use of condensation pressure control wherein the condensation pressure of the external heat-exchanger is controlled within a prescribed range by regulating the rotation speed of the external fan is not practically effective for cooling. In general, changes in condensation pressure of the external heat-exchanger substantially correspond to changes in condensation temperature of the external heat-exchanger. As shown in FIG. 1, the condensation temperature of external heat-exchanger 15 is detected by sensor 27 disposed at the discharge side of external heat-exchanger 15. When external temperature decreases below a prescribed level, changes in condensation temperature of the external heat-exchanger do not correspond to changes in condensation pressure thereof. This is because a liquidized refrigerant tends to stay at a portion of the pipe where sensor 39 is disposed. Furthermore, refrigerant at the portion of pipe where sensor 39 is disposed is in a supercooling state, as indicated by point A in FIG. 3, which shows a mollier diagram. According to FIG. 3, the condensation temperature of external heat-exchanger 15 is constant even if the condensation pressure changes in the supercooling area. Therefore, it is suitable to detect a defrost state when sensor 39 is disposed on the pipe of the other end (discharge side during cooling) of the lower-most side passage element 31c. However, it is unsuitable to use the output of sensor 39 for controlling the rotation speed of the external fan. If such an output is used for controlling the rotation speed of the external fan during cooling in the spring or fall, an undesirable reduction in condensation pressure occurs when the external temperature decreases. Thus, a sufficient throttle action is not performed by the decompression device, and liquidized refrigerant returns into the compressor. As a result, damage of the compressor is caused by liquid compression. A shortage of lubricating oil in the compressor also occurs because of the mixing of lubricating oil into the liquidized refrigerant.